1. Field of the Invention
The present invention relates to heating elements and more particularly to heating elements comprising a ceramic article of a honeycomb structure having a positive temperature coefficient of electric resistance (hereinafter referred to as a PTC ceramic article) and composed of barium titanate, which is provided with a pair of ohmic electrodes consisting mainly of silver as a metallic component on both end surfaces of the ceramic honeycomb.
In the remainder of the specification, the term "honeycomb structure" shall be understood to mean a structure having a multiplicity of parallel channels extending therethrough with each of said channels being bounded by a partition wall which is substantially uniform in thickness, and having a surface-to-volume ratio in the range of 10 to 60 cm.sup.2 /cm.sup.3.
2. Description of the Prior Art
Heretofore, a baked paste consisting mainly of silver as a metallic component has been generally known as the ohmic electrode for a plate-shaped PTC ceramic article composed of barium titanate.
U.S. Pat. No. 3,927,300 granted on Dec. 16, l975 to S. Wada et al has disclosed a heating element in which a pair of ohmic electrodes composed of the conventional baked silver paste are provided on the opposite end surfaces 3 and 3' of PTC ceramic article 1 of a honeycomb structure composed of barium titanate and having a multiplicity of parallel channels 2 extending therethrough as shown in FIG. 1, but there has been a defect that in this heating element, the resistance of the ohmic electrodes per se increases during flowing electric current and cracks are formed on the electrodes and the ohmic electrodes are fused and broken in a short time and such a heating element has not been practically used.
Therefore, the inventors have proposed in the above described patent that aluminum be hot sprayed on the opposite end surfaces 3 and 3' of the channels in PTC ceramic article 1 of the honeycomb structure composed of barium titanate to form the ohmic electrodes. But with aluminum it is difficult to provide a lead wire, and in the hot spraying process, the sprayed aluminum is adhered to the inner portion of the channels 2 and the cross-sectional area of the channels becomes small, this process is not preferable.
Furthermore, the above described U.S. patent discloses that a corrosion resistant material is coated on the whole heating element, but this only attempts electrical insulation against the electricly conductive fluid to be heated, such as water and such a disclosure has no relation to the subject matter of the present invention.